DISEASES OF THE HORSE. 277 



tors, the pyramidal and orbicular, the digastricus, the vastus, and so 

 on. Those which are under the control of the will, known arj the vol- 

 untary muscles, appear in the form of fleshy structures, red in color, 

 and with fibers of various degrees of fineness, and are composed of 

 fascicuU., or bundles of fibers, united by connective or cellular tis- 

 sue, each fasciculus being composed of smaller ones, less in size, but 

 united in a similar manner to compose the larger formations, each of 

 which is enveloped bv a structure of similar nature known as the mr- 

 colemma. jVIan}- of the muscles are united to the bones b}' the direct 

 contact of their fleshy fibers, but, in other instances, the bod}^ of the 

 muscle is more or less gradually transformed into a cordy or mem- 

 branous structure known as the tendon or sinew, and the attachment 

 is m.ade by very short fibrous threads through the medium of a long 

 tendinous band, which, passing from a single one to several other of 

 the bones, effects its object at a point far distant from its original 

 attachment. In thus carrying its action from one bone to another, or 

 from one region of a limb to another, these tendons must necessarily 

 have smooth surfaces over which to glide, either upon the bones them- 

 selves or formed at their articulations, and this need is supplied b}^ 

 the secretion of the synovial fluid, a yellowish, unctuous substance, 

 furnished by a peculiar tendinous synovial sac, designed for the 

 purpose. 



Illustrations in point of the agency of the synovial fluid in assisting 

 the sliding movements of the tendons may be found under their vari- 

 ous forms at the shoulder joint, at the upper part of the bone of the 

 arm, at the posterior part of the kneejoint, and also at the fetlocks, 

 on their posterior part. 



As the tendons, whether singly or in company with others, pass 

 over these natural puller's they are retained in place b}- strong fibrous 

 bands or sheaths, which are by no means exempt from danger of 

 injury, as will be readily inferred from a consideration of their impor- 

 tant special use as supports and reenforcements of the tendons them- 

 selves, with which they must necessarily share the stress of whatever 

 force or strain is brought to bear upon both or either. 



We have referred to that special formation of the external surface 

 of a bone by which it is adapted to form a joint or articulation, either 

 movable or fixed, and a concise examination of the formation and 

 structure of the movable articulations will here be in place. These 

 are formed generally by the extremities of the long, or ma}' exist on the 

 surfaces of the short, bones. The points or regions where the contact 

 occurs is denominated the articular surface, which assumes from this 

 circumstance a considerable variety of aspect and form, being in one 

 case comparative!}- flat and another elevated; or as forming a protrud- 

 ing head or knob, with a distinct convexity; and again presenting a 

 corresponding depression or cavit}', accurately adapted to complete, 



